I haven't filed my taxes in two years but I still received last April's stimulus check. However, I have received the second or third stimulus. Why?

The IRS used your 2018 income tax return for the first stimulus check. 

To claim the remainder of your recovery rebate credit (stimulus), see How do I claim the recovery rebate if I don’t have taxable income?

To claim the Recovery Rebate Credit, you

• Must be owed an additional amount based on the information in this year’s 2020 Tax Filing. The maximum amounts are:
  • First payment: $1,200 ($2,400 if married filing jointly for 2020) plus $500 for each qualifying child you had in 2020; and
  • Second payment: $600 ($1,200 if married filing jointly for 2020) plus $600 for each qualifying child you had in 2020
• Must be a U.S. citizen or U.S. resident alien,
• Have a valid Social Security number, and
• You can’t have been claimed as a dependent on someone else’s 2020 tax return